By Nick Triggle
BBC News health reporter
Fear about bird flu is gripping Europe after the deadly influenza strain reached the continent last week.
One million people died in the Asian flu pandemic
And now experts are warning if it mutates with a human version it could kill 50,000 in the UK alone.
But if that does happen, it will not be the first time the world has had to do battle with such a deadly flu.
During the last century there were three pandemics.
So how were they dealt with and what
are the lessons for today?
The first and worst flu pandemic started in 1918. The so-called Spanish flu killed 40 million people - more than died in World War I which was coming to an end at the time.
The flu took the world by surprise and authorities were slow to react, said Professor David Killingray, a historian at Goldsmith's College in London.
"Countries were really not prepared. They did not really know it was a virus which was causing all the deaths and instead concentrated on bacteria.
"Vaccines were developed, but it was a shotgun approach. People were pumped full of lots of things, which probably did more harm than good."
Countries such as the UK and the United States also relied on quarantine and good personal hygiene.
Spanish Flu 1918-9 - Killed up to 40m as authorities struggled to cope at the tail end of the First World War. People put in quarantine, encouraged to keep high levels of personal hygiene. But vaccines were targeted at bacteria rather than a virus
Asian Flu 1957-8 - About 1m died from the flu, but medical response had moved on somewhat since 1918. Strain was identified and vaccines developed, but not enough of them. Quarantine again used to little effect
Hong Kong Flu 1968-9 - Similar death toll to the Asian Flu. Vaccines developed, but did not meet demand
Schools and cinemas were closed to stop the spread of the virus and in some cases people suspected of having the virus were forced into isolated hospital wards.
People were also encouraged to wash their hands and disinfect their living environments.
But Professor Michael Worboys, director of the Centre for the History of Science, Technology and Medicine, said: "These measures were not really that successful, troops were moving all over the world and the flu quickly spread."
The situation was completely different by the time of the next pandemic in 1957.
By then the World Health Organisation had set up a global flu surveillance network which was able to give an early warning that an outbreak was imminent when the virus started spreading across south-east Asia at the end of February.
Within months scientists in WHO labs in Japan and Singapore had identified the strain and health services across the world braced themselves.
Vaccines were developed in western countries, but not enough could be produced. Again isolation and quarantine was used. But to little success.
In the end one million people died - less than in 1918 but much of that could be put down to it being a milder strain.
Professor Worboys said another difference was the availability of universal health care in the UK.
The NHS had been set up nine years before, and unlike 1918 people had access to doctors when they became ill.
"Prior to the NHS only working men had any sort of health insurance. It meant that poorer people, especially women and children, did not use health services so in the most deprived pockets of the country plenty of people died in 1918."
It was a similar story in 1968 when Hong Kong flu emerged.
Again the WHO made warnings, again vaccines were produced. But still there was not enough - the death toll hit one million.
But that has not stopped scientists wanting to learn from the past.
Researchers for the Centers for Disease Control and Prevention, part of the US government, have reconstructed the 1918 virus to try to understand more about it.
CDC director Dr Julie Gerberding said such research was essential as there was much more that needed to be understood about flu viruses.
Dr Abigail Woods, a lecturer in the history of medicine at Imperial College London, agrees we ignore history at our peril.
"Lessons can be learned from the past. Although you cannot apply the measures taken then strictly to what is happening today.
Bird flu has reached Europe
"Analysing how it spread and what happened with vaccines can still help."
However, Dr Woods said, as the 2005 flu is being carried by birds, the government should also be looking for more recent lessons.
"The foot-and-mouth outbreak offers a guide to how the movement of animals means disease can spread quickly. I think that is better understood now than it was a few years ago."
And coupled with that, nearly 40 years on since the last flu pandemic, technology has moved on as well.
Surveillance is much more advanced. The flu threat has been identified before it can even be spread from human to human - the H5N1 strain can only be passed from bird to human at the moment.
Such an early warning system means measures such as bird culling can be taken to prevent a pandemic even starting - as they already have done.
The WHO believes a pandemic was prevented in 1997 when Hong Kong authorities killed the entire 1.5 million poultry population after 18 humans had developed the bird flu.
Launching a recent report on flu preparedness, WHO director general Dr Lee Jong-wook said: "In the past, pandemics have announced themselves with a sudden explosion of cases which took the world by surprise.
"This time, we have been given a clear warning."
The early warning has allowed scientists to develop drugs to help treat and prevent bird flu - millions of doses of antiviral drug Tamiflu has been ordered by governments across the world - but the effectiveness of these is somewhat limited.
However, like the earlier epidemics, the battle is still on for a vaccine.
Experts have said a jab cannot be produced until the H5N1 strain mutates.
And even then it will take at least four months to produce.
The challenge, it seems, is still the same as is was in the last pandemic. Are countries able to produce enough vaccine to prevent mass casualties?